CalendarDateToISO: Constrain behaviour for regnal years?

[anba]

It'd be nice to have some more detailed information how constraining should work for regnal years in the Japanese calendar.

1. Dates before the start of the era (from SpiderMonkey WIP implementation):

js> Temporal.PlainDate.from({calendar: "japanese", era: "reiwa", eraYear: 1, month: 1, day: 1}).toString()
"2019-05-01[u-ca=japanese]"
js> Temporal.PlainDate.from({calendar: "japanese", era: "reiwa", eraYear: 1, month: 1, day: 1}).era
"reiwa"
js> Temporal.PlainDate.from({calendar: "japanese", era: "reiwa", eraYear: 1, month: 1, day: 1}, {overflow: "reject"}).toString()
typein:1:20 RangeError: calendar field "eraYear" is too large: 1

Whereas the spec polyfill changes to the previous era (for both constrain and reject overflow behaviour):

js> Temporal.PlainDate.from({calendar: "japanese", era: "reiwa", eraYear: 1, month: 1, day: 1}).toString()
"2019-01-01[u-ca=japanese]"
js> Temporal.PlainDate.from({calendar: "japanese", era: "reiwa", eraYear: 1, month: 1, day: 1}).era       
"heisei"

2. Dates after the end of the era (from SpiderMonkey WIP implementation):

js> Temporal.PlainDate.from({calendar: "japanese", era: "heisei", eraYear: 100, month: 1, day: 1}).toString()
"2019-04-30[u-ca=japanese]"
js> Temporal.PlainDate.from({calendar: "japanese", era: "heisei", eraYear: 100, month: 1, day: 1}).era
"heisei"
js> Temporal.PlainDate.from({calendar: "japanese", era: "heisei", eraYear: 100, month: 1, day: 1}, {overflow: "reject"}).toString()
typein:1:20 RangeError: calendar field "eraYear" is too large: 100

Spec polyfill has (for both constrain and reject overflow behaviour):

js> Temporal.PlainDate.from({calendar: "japanese", era: "heisei", eraYear: 100, month: 1, day: 1}).toString()
"2088-01-01[u-ca=japanese]"
js> Temporal.PlainDate.from({calendar: "japanese", era: "heisei", eraYear: 100, month: 1, day: 1}).era       
"reiwa"

Questions I've asked myself:

• Does overflow behaviour affect era/eraYear?
• If yes, does constrain overflow constrain to the start/end of the era?
• If yes, does reject overflow reject dates before/after the era?

[justingrant]

Meeting 2024-05-30:

• For constrain we'll treat each era "proleptically": dates before the era starts are mapped to the previous era (without changing the ISO date due to constraining), and dates after the era ends are mapped to later era (again without changing ISO date).
• For reject, throw if it's before or after the era boundaries.

@gibson042 to make a PR. Because current behavior is unspecified, this will not be a normative change.

[anba]

I've just found tc39/ecma402#540 which actually wants a different behaviour for reject, namely that reject doesn't throw for dates after the end of the era. I don't know if this should imply that reject should also not throw for dates before the start of the era, though.